In the optical arts, it is often necessary to fabricate an optical component with a reflective optical surface. For example, a workpiece of optical material is mounted in a machine tool, and the tool is then used to carry out a diamond point turning operation that forms an optical surface on the workpiece. For some applications, the reflective optical surface is a conic surface, such as an ellipsoid, or paraboloid of revolution.
After the optical surface has been created, it is usually tested for accuracy, for example by using an interferometer. For test purposes, the optical surface needs to be mechanically aligned very accurately with respect to the interferometer. This is usually achieved by making a very precise test jig or fixture that mates with some mechanical feature on the workpiece. However, these test jigs or fixtures are relatively expensive, and can suffer from a build up of tolerances. Consequently, although pre-existing testing techniques and devices have been generally adequate for their intended purposes, they have not been satisfactory in all respects.